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上一章节中,我们分析了Netty服务的启动过程,本章节分析Netty的NioEventLoop是如工作的。

img

NioEventLoop中维护了一个线程,线程启动时会调用NioEventLoop的run方法,执行I/O任务和非I/O任务:

I/O任务
即selectionKey中ready的事件,如accept、connect、read、write等,由processSelectedKeys方法触发。

非IO任务
添加到taskQueue中的任务,如register0、bind0等任务,由runAllTasks方法触发。

两种任务的执行时间比由变量ioRatio控制,默认为50,则表示允许非IO任务执行的时间与IO任务的执行时间相等。

NioEventLoop.run 方法实现

protected void run() {
for (;;) {
boolean oldWakenUp = wakenUp.getAndSet(false);
try {
if (hasTasks()) {
selectNow();
} else {
select(oldWakenUp);
if (wakenUp.get()) {
selector.wakeup();
}
}

cancelledKeys = 0;
needsToSelectAgain = false;
final int ioRatio = this.ioRatio;
if (ioRatio == 100) {
processSelectedKeys();
runAllTasks();
} else {
final long ioStartTime = System.nanoTime();

processSelectedKeys();

final long ioTime = System.nanoTime() - ioStartTime;
runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
}

if (isShuttingDown()) {
closeAll();
if (confirmShutdown()) {
break;
}
}
} catch (Throwable t) {
logger.warn("Unexpected exception in the selector loop.", t);

// Prevent possible consecutive immediate failures that lead to
// excessive CPU consumption.
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// Ignore.
}
}
}
}

hasTasks()方法判断当前taskQueue是否有元素。
1、 如果taskQueue中有元素,执行 selectNow() 方法,最终执行selector.selectNow(),该方法会立即返回。

void selectNow() throws IOException {
try {
selector.selectNow();
} finally {
// restore wakup state if needed
if (wakenUp.get()) {
selector.wakeup();
}
}
}

2、 如果taskQueue没有元素,执行 select(oldWakenUp) 方法,代码如下:

private void select(boolean oldWakenUp) throws IOException {
Selector selector = this.selector;
try {
int selectCnt = 0;
long currentTimeNanos = System.nanoTime();
long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);
for (;;) {
long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
if (timeoutMillis <= 0) {
if (selectCnt == 0) {
selector.selectNow();
selectCnt = 1;
}
break;
}

int selectedKeys = selector.select(timeoutMillis);
selectCnt ++;

if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
// - Selected something,
// - waken up by user, or
// - the task queue has a pending task.
// - a scheduled task is ready for processing
break;
}
if (Thread.interrupted()) {
// Thread was interrupted so reset selected keys and break so we not run into a busy loop.
// As this is most likely a bug in the handler of the user or it's client library we will
// also log it.
//
// See https://github.com/netty/netty/issues/2426
if (logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely because " +
"Thread.currentThread().interrupt() was called. Use " +
"NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");
}
selectCnt = 1;
break;
}

long time = System.nanoTime();
if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
// timeoutMillis elapsed without anything selected.
selectCnt = 1;
} else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
// The selector returned prematurely many times in a row.
// Rebuild the selector to work around the problem.
logger.warn(
"Selector.select() returned prematurely {} times in a row; rebuilding selector.",
selectCnt);

rebuildSelector();
selector = this.selector;

// Select again to populate selectedKeys.
selector.selectNow();
selectCnt = 1;
break;
}

currentTimeNanos = time;
}

if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS) {
if (logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely {} times in a row.", selectCnt - 1);
}
}
} catch (CancelledKeyException e) {
if (logger.isDebugEnabled()) {
logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector - JDK bug?", e);
}
// Harmless exception - log anyway
}
}

这个方法解决了Nio中臭名昭著的bug:selector的select方法导致cpu100%。
1、delayNanos(currentTimeNanos):计算延迟任务队列中第一个任务的到期执行时间(即最晚还能延迟多长时间执行),默认返回1s。每个SingleThreadEventExecutor都持有一个延迟执行任务的优先队列PriorityQueue,启动线程时,往队列中加入一个任务。

protected long delayNanos(long currentTimeNanos) {
ScheduledFutureTask<?> delayedTask = delayedTaskQueue.peek();
if (delayedTask == null) {
return SCHEDULE_PURGE_INTERVAL;
}
return delayedTask.delayNanos(currentTimeNanos);
}

//ScheduledFutureTask
public long delayNanos(long currentTimeNanos) {
return Math.max(0, deadlineNanos() - (currentTimeNanos - START_TIME));
}
public long deadlineNanos() {
return deadlineNanos;
}

2、如果延迟任务队列中第一个任务的最晚还能延迟执行的时间小于500000纳秒,且selectCnt == 0(selectCnt 用来记录selector.select方法的执行次数和标识是否执行过selector.selectNow()),则执行selector.selectNow()方法并立即返回。
3、否则执行selector.select(timeoutMillis),这个方法已经在深入浅出NIO Socket分析过。
4、如果已经存在ready的selectionKey,或者selector被唤醒,或者taskQueue不为空,或则scheduledTaskQueue不为空,则退出循环。
5、如果 selectCnt 没达到阈值SELECTOR_AUTO_REBUILD_THRESHOLD(默认512),则继续进行for循环。其中 currentTimeNanos 在select操作之后会重新赋值当前时间,如果selector.select(timeoutMillis)行为真的阻塞了timeoutMillis,第二次的timeoutMillis肯定等于0,此时selectCnt 为1,所以会直接退出for循环。
6、如果触发了epool cpu100%的bug,会发生什么?
selector.select(timeoutMillis)操作会立即返回,不会阻塞timeoutMillis,导致 currentTimeNanos 几乎不变,这种情况下,会反复执行selector.select(timeoutMillis),变量selectCnt 会逐渐变大,当selectCnt 达到阈值,则执行rebuildSelector方法,进行selector重建,解决cpu占用100%的bug。

public void rebuildSelector() {
if (!inEventLoop()) {
execute(new Runnable() {
@Override
public void run() {
rebuildSelector();
}
});
return;
}
final Selector oldSelector = selector;
final Selector newSelector;
if (oldSelector == null) {
return;
}
try {
newSelector = openSelector();
} catch (Exception e) {
logger.warn("Failed to create a new Selector.", e);
return;
}
// Register all channels to the new Selector.
int nChannels = 0;
for (;;) {
try {
for (SelectionKey key: oldSelector.keys()) {
Object a = key.attachment();
try {
if (key.channel().keyFor(newSelector) != null) {
continue;
}
int interestOps = key.interestOps();
key.cancel();
key.channel().register(newSelector, interestOps, a);
nChannels ++;
} catch (Exception e) {
logger.warn("Failed to re-register a Channel to the new Selector.", e);
if (a instanceof AbstractNioChannel) {
AbstractNioChannel ch = (AbstractNioChannel) a;
ch.unsafe().close(ch.unsafe().voidPromise());
} else {
@SuppressWarnings("unchecked")
NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
invokeChannelUnregistered(task, key, e);
}
}
}
} catch (ConcurrentModificationException e) {
// Probably due to concurrent modification of the key set.
continue;
}

break;
}
selector = newSelector;
try {
// time to close the old selector as everything else is registered to the new one
oldSelector.close();
} catch (Throwable t) {
if (logger.isWarnEnabled()) {
logger.warn("Failed to close the old Selector.", t);
}
}
logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
}

rebuildSelector过程:
1、通过方法openSelector创建一个新的selector。
2、将old selector的selectionKey执行cancel。
3、将old selector的channel重新注册到新的selector中。

对selector进行rebuild后,需要重新执行方法selectNow,检查是否有已ready的selectionKey。

方法selectNow()或select(oldWakenUp)返回后,执行方法processSelectedKeys和runAllTasks。
1、processSelectedKeys 用来处理有事件发生的selectkey,这里对优化过的方法processSelectedKeysOptimized进行分析:

private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {
for (int i = 0;; i ++) {
final SelectionKey k = selectedKeys[i];
if (k == null) {
break;
}
// null out entry in the array to allow to have it GC'ed once the Channel close
// See https://github.com/netty/netty/issues/2363
selectedKeys[i] = null;

final Object a = k.attachment();

if (a instanceof AbstractNioChannel) {
processSelectedKey(k, (AbstractNioChannel) a);
} else {
@SuppressWarnings("unchecked")
NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
processSelectedKey(k, task);
}

if (needsToSelectAgain) {
// null out entries in the array to allow to have it GC'ed once the Channel close
// See https://github.com/netty/netty/issues/2363
for (;;) {
i++;
if (selectedKeys[i] == null) {
break;
}
selectedKeys[i] = null;
}

selectAgain();
// Need to flip the optimized selectedKeys to get the right reference to the array
// and reset the index to -1 which will then set to 0 on the for loop
// to start over again.
//
// See https://github.com/netty/netty/issues/1523
selectedKeys = this.selectedKeys.flip();
i = -1;
}
}
}

在优化过的方法中,有事件发生的selectkey存放在数组selectedKeys中,通过遍历selectedKeys,处理每一个selectkey,具体处理过程,会在后续进行分析。

2、runAllTasks 处理非I/O任务。
如果 ioRatio 不为100时,方法runAllTasks的执行时间只能为ioTime * (100 - ioRatio) / ioRatio,其中ioTime 是方法processSelectedKeys的执行时间。

protected boolean runAllTasks(long timeoutNanos) {
fetchFromScheduledTaskQueue();
Runnable task = pollTask();
if (task == null) {
return false;
}

final long deadline = ScheduledFutureTask.nanoTime() + timeoutNanos;
long runTasks = 0;
long lastExecutionTime;
for (;;) {
try {
task.run();
} catch (Throwable t) {
logger.warn("A task raised an exception.", t);
}
runTasks ++;
// Check timeout every 64 tasks because nanoTime() is relatively expensive.
// XXX: Hard-coded value - will make it configurable if it is really a problem.
if ((runTasks & 0x3F) == 0) {
lastExecutionTime = ScheduledFutureTask.nanoTime();
if (lastExecutionTime >= deadline) {
break;
}
}
task = pollTask();
if (task == null) {
lastExecutionTime = ScheduledFutureTask.nanoTime();
break;
}
}
this.lastExecutionTime = lastExecutionTime;
return true;
}

方法fetchFromScheduledTaskQueue把scheduledTaskQueue中已经超过延迟执行时间的任务移到taskQueue中等待被执行。

private void fetchFromScheduledTaskQueue() {
if (hasScheduledTasks()) {
long nanoTime = AbstractScheduledEventExecutor.nanoTime();
for (;;) {
Runnable scheduledTask = pollScheduledTask(nanoTime);
if (scheduledTask == null) {
break;
}
taskQueue.add(scheduledTask);
}
}
}

依次从taskQueue任务task执行,每执行64个任务,进行耗时检查,如果已执行时间超过预先设定的执行时间,则停止执行非IO任务,避免非IO任务太多,影响IO任务的执行。

END。
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